Chapter 10: Blood – Structure, Function, and Clinical Relevance

Blood: Composition and Functions

Overview of Blood

Blood is the only fluid tissue in the human body and is classified as a connective tissue. It plays a vital role in transporting substances, regulating physiological parameters, and protecting the body against disease.

• Components of Blood:

  • Formed elements (living cells): erythrocytes, leukocytes, and platelets

  • Plasma (non-living matrix): the liquid portion containing dissolved substances

Physical Characteristics of Blood

• Color: Oxygen-rich blood is scarlet red; oxygen-poor blood is dull red.

• pH: Must remain between 7.35–7.45 for proper physiological function.

• Volume: In a healthy adult male, blood volume is about 5–6 liters (6 quarts), making up approximately 8% of body weight.

Blood Composition After Centrifugation

• Erythrocytes (RBCs): Settle at the bottom (45% of blood, known as the hematocrit).

• Buffy coat: Thin, whitish layer between erythrocytes and plasma; contains leukocytes and platelets (<1% of blood).

• Plasma: Rises to the top (55% of blood).

Blood Plasma

Composition and Functions

Blood plasma is a straw-colored, sticky fluid that makes up about 55% of blood volume. It is composed of approximately 90% water and contains a variety of dissolved substances.

• Nutrients: Glucose, amino acids, fatty acids, vitamins

• Salts (Electrolytes): Sodium, potassium, calcium, magnesium, chloride, bicarbonate

• Respiratory Gases: Oxygen and carbon dioxide

• Hormones

• Plasma Proteins: Albumin, clotting proteins, antibodies

• Waste Products: Urea, uric acid, creatinine

Plasma Proteins

• Albumin: Regulates osmotic pressure, maintaining blood volume and pressure.

• Clotting Proteins: Help stem blood loss when a blood vessel is injured (e.g., fibrinogen, prothrombin).

• Antibodies (Immunoglobulins): Protect the body from pathogens.

Formed Elements of Blood

Overview

• Erythrocytes (Red Blood Cells, RBCs): Specialized for oxygen transport.

• Leukocytes (White Blood Cells, WBCs): Crucial for immune defense.

• Platelets (Thrombocytes): Cell fragments essential for blood clotting.

Table: Characteristics of Formed Elements of the Blood

Cell Type	Occurrence in Blood (per mm3)	Cell Anatomy	Function
Erythrocytes (RBCs)	4–6 million	Biconcave, anucleate, essentially bags of hemoglobin	Transport oxygen bound to hemoglobin; small amount of CO2 transport
Leukocytes (WBCs)	4,000–11,000	Complete cells with nucleus and organelles	Defense against disease
Platelets	150,000–400,000	Cell fragments from megakaryocytes	Needed for blood clotting

Erythrocytes (Red Blood Cells)

• Main Function: Carry oxygen from the lungs to tissues.

• Structure: Biconcave disks, anucleate, very few organelles, filled with hemoglobin.

• Hemoglobin: Iron-containing protein; each molecule binds up to four oxygen molecules. Each RBC contains about 250 million hemoglobin molecules.

• Normal Hemoglobin Content: 12–18 g per 100 mL of blood.

Homeostatic Imbalances of RBCs

• Anemia: Decreased oxygen-carrying capacity of blood due to low RBC count or abnormal hemoglobin.

• Sickle Cell Anemia (SCA): Genetic disorder causing abnormally shaped hemoglobin, leading to sickle-shaped RBCs.

• Polycythemia: Excessive or abnormal increase in RBC number, increasing blood viscosity.

Table: Types of Anemia

Direct Cause	Resulting From	Leading To
Decrease in RBC number	Hemorrhage, hemolysis, lack of vitamin B12, destruction of bone marrow	Hemorrhagic, hemolytic, pernicious, aplastic anemia
Inadequate hemoglobin content in RBCs	Lack of iron or slow/bleeding ulcers	Iron-deficiency anemia
Abnormal hemoglobin in RBCs	Genetic defects (e.g., sickle cell gene)	Sickle cell anemia

Leukocytes (White Blood Cells)

• Function: Crucial in defense against disease; can move in and out of blood vessels (diapedesis) and respond to tissue damage.

• Normal Count: 4,000–11,000 per mm3 of blood.

• Abnormal Numbers:

  • Leukocytosis: WBC count above 11,000/mm3, usually indicates infection.

  • Leukopenia: Abnormally low WBC count, often due to drugs.

  • Leukemia: Cancerous bone marrow produces excess, abnormal WBCs.

Types of Leukocytes

• Granulocytes: Contain granules in cytoplasm, lobed nuclei.

  • Neutrophils: Phagocytes at infection sites.

  • Eosinophils: Respond to allergies and parasitic worms.

  • Basophils: Contain histamine; initiate inflammation.

• Agranulocytes: Lack visible granules, nuclei are spherical/oval/kidney-shaped.

  • Lymphocytes: Important in immune response (B and T cells).

  • Monocytes: Largest WBCs; become macrophages, fight chronic infection.

Relative Abundance of WBCs

• Neutrophils > Lymphocytes > Monocytes > Eosinophils > Basophils

• Mnemonic: Never Let Monkeys Eat Bananas

Platelets

• Origin: Derived from ruptured megakaryocytes (large multinucleate cells).

• Function: Essential for blood clotting (hemostasis).

• Normal Count: 150,000–400,000 per mm3 (average 300,000/mm3).

Blood Groups and Transfusions

Importance of Blood Groups

Blood transfusions are critical for replacing lost blood, but compatibility is essential to prevent immune reactions. Blood groups are determined by genetically inherited antigens on RBC surfaces.

• Major Blood Groups: ABO and Rh systems

• Antigens: Substances recognized as foreign by the immune system; can trigger antibody production.

• Antibodies: Proteins that recognize and bind to specific antigens, causing agglutination (clumping) if incompatible blood is transfused.

Table: ABO Blood Groups

Blood Group	RBC Antigens	Plasma Antibodies	Blood That Can Be Received
AB	A, B	None	A, B, AB, O (Universal recipient)
B	B	Anti-A	B, O
A	A	Anti-B	A, O
O	None	Anti-A, Anti-B	O (Universal donor)

ABO Blood Group System

• Type AB: Both A and B antigens; no anti-A or anti-B antibodies; universal recipient.

• Type A: A antigen; anti-B antibodies.

• Type B: B antigen; anti-A antibodies.

• Type O: No antigens; both anti-A and anti-B antibodies; universal donor.

Rh Blood Group System

• Rh Antigen (D): Presence (+) or absence (−) of Rh antigen on RBCs.

• Most Americans are Rh positive (Rh+).

• Transfusion Reaction: Mixing Rh+ blood into an Rh− recipient can cause immune complications.

Rh Dangers During Pregnancy

• Occurs when an Rh− mother carries an Rh+ fetus.

• First pregnancy usually proceeds without problems; the mother becomes sensitized.

• In subsequent pregnancies, maternal anti-Rh antibodies can attack fetal RBCs, causing hemolytic disease of the newborn.

• Prevention: RhoGAM injection prevents maternal sensitization.

Blood Typing and Cross-Matching

Blood Typing Procedure

• Blood samples are mixed with anti-A and anti-B sera.

• Coagulation (agglutination) indicates the presence of the corresponding antigen.

• Typing for ABO and Rh factors is performed similarly.

• Cross-matching tests for agglutination of donor RBCs by recipient serum and vice versa.

Example: Blood Typing Results

• Type AB: Agglutinates with both anti-A and anti-B sera.

• Type A: Agglutinates with anti-A serum only.

• Type B: Agglutinates with anti-B serum only.

• Type O: No agglutination with either serum.

Summary Table: Blood Groups, Antigens, Antibodies, and Compatibility

Blood Type	Antigens on RBCs	Antibodies in Plasma	Can Donate To	Can Receive From
A	A	Anti-B	A, AB	A, O
B	B	Anti-A	B, AB	B, O
AB	A, B	None	AB	All types
O	None	Anti-A, Anti-B	All types	O

Key Equations and Concepts

• Hematocrit: Percentage of RBCs in total blood volume.

  • $\text{Hematocrit} = \frac{\text{Volume of RBCs}}{\text{Total Blood Volume}} \times 100\%$

Clinical Application Example

• Blood Transfusion: A patient with type B blood can safely receive type B or type O blood, but not type A or AB, to avoid agglutination and transfusion reactions.

• Hemolytic Disease of the Newborn: Prevented by administering RhoGAM to Rh− mothers after delivery of an Rh+ baby.

Additional info: This guide expands on the provided slides and notes with definitions, clinical context, and tables for clarity and exam preparation.